This invention relates to color liquid crystal displays and, in particular, to a technique for backlighting a color liquid crystal display.
Liquid crystal displays (LCDs) are commonly used in battery operated equipment, such as cell phones, personal digital assistants (PDAs), and laptop computers, and are becoming popular for desktop and television applications, where they replace bulky CRTs. Presently, drawbacks of such LCDs include limited brightness, low efficiency, and limited viewing angle. LCDs can be monochrome or color and can be transmissive or reflective. The present invention deals with a color, transmissive LCD that requires backlighting, where the backlighting contains red, green, and blue components.
FIG. 1 is a cross-sectional view of a small portion of a prior art color, transmissive LCD. There are other types of color, transmissive LCD structures. The structure of FIG. 1 will be used to identify certain disadvantages of prior art LCDs that are avoided by the present invention.
In FIG. 1, an LCD 10 includes a white light source 12 to provide backlighting for the upper LCD layers. A common source for white light is a fluorescent bulb. Another white light source is a combination of red, green, and blue light emitting diodes (LEDs) whose combined light forms white light. Other white light sources are known. These white light sources must provide homogeneous light to the back surface of the display. A popular technique for providing such a homogeneous white light is to optically couple the fluorescent bulb or LEDs to a light guide, such as by optically coupling the light source to one or more edges of a sheet of clear plastic. The sheet has deformities that bend the light approximately normal to the top surface of the sheet so that light is emitted from the top surface. Examples of such deformities include ridges in the bottom surface, reflective particles embedded into the plastic sheet, or a roughening of the top or bottom surface of the sheet. The deformities cause a quasi-uniform plane of light to be emitted out the front surface of the light guide. A reflector may be placed behind the back surface of the light guide to improve brightness and uniformity.
It is also common to not use any light guide, wherein a light source positioned behind the display is provided with appropriate diffusers to uniformly distribute the light across the display.
A polarizing filter 14 linearly polarizes the white light. In the embodiment shown in FIG. 1, the polarizing filter 14 is formed in a glass substrate having transparent conductors.
Above the polarizing filter 14 is a liquid crystal layer 16, and above liquid crystal layer 16 is a glass substrate 18 having transparent conductors. Selected conductors in the glass substrates are energized by display control signals coupled to the electrodes 19, 20. The absence of an electrical field across a pixel area of the liquid crystal layer 16 causes light passing through that pixel area to have its polarization rotated orthogonal to the incoming polarization. An electrical field across a pixel area of the liquid crystal layer 16 causes the liquid crystals to align and not affect the polarity of light. Selectively energizing the conductors controls the localized electric fields across the liquid crystal layer 16. Both normally open (white) and normally closed (black) shutters are used in different displays.
Instead of a passive conductor array, a transparent thin film transistor (TFT) array may be used, having one transistor for each pixel. TFT arrays are extremely well known and need not be further described.
The light output from the glass substrate 18 is then filtered by an RGB pixel filter 22. The RGB pixel filter 22 may be comprised of a red filter layer, a green filter layer, and a blue filter layer. The layers may be deposited as thin films. As an example, the red filter contains an array of red light filter areas coinciding with the red pixel areas of the display. The remaining portions of the red filter are clear to allow other light to pass. Accordingly, the RGB pixel filter 22 provides a filter for each R, G, and B pixel in the display.
A polarizing filter 24 only passes polarized light orthogonal to the light output from the polarizing filter 14. Therefore, the polarizing filter 24 only passes light that has been polarized by a non-energized pixel area in the liquid crystal layer 16 and absorbs all light that passes through the energized portions of the liquid crystal layer 16. The magnitudes of the electric fields across the liquid crystal layer 16 control the brightness of the individual R, G, and B components to create any color. In this manner, any color image may be presented to the viewer by selectively energizing the various conducters.
The RGB pixel filter 22 inherently filters two-thirds of all light reaching it, since each filter only allows one of the three primary colors to pass. This is a significant factor in the generally poor efficiency of the prior art LCDs. The overall transmissivity of the LCD layers above the white light source 12 is on the order of 4-10%. What is needed is a technique for increasing the brightness of an LCD output without requiring additional energy for the white light source.
FIG. 2 illustrates another prior art color LCD. The layer labeled LCD layers 28 may include all the layers in FIG. 1 except for the RGB pixel filter 22 or may be any other layers for implementing an LCD. FIG. 2 does not use a white light source but instead sequentially energizes red, green, and blue light sources 30, such as red, green, and blue LEDs. A light guide 32 typically receives the RGB light along one or more of its edges and bends the light toward the LCD layers 28 using any one of a number of well known techniques. Sequentially energizing the RGB light sources requires synchronization with the energization of the TFT array. Additionally, to avoid any perceivable flicker, the R, G, and B light sources must each be energized at a frequency of at least 180 Hz to accommodate all three colors sequentially at 60 frames per second. The switching speed may need to be even faster to account for motion artifacts such as those caused by the viewer moving his head while viewing the display. Problems with slow switching speed of the shutter (LC+TFT) and motion artifacts will likely keep this approach impractical for at least several more years.
A color, transmissive LCD is described herein which uses red, green, and blue LEDs as the light source. The R, G, and B LEDs are coupled to separate light guides, one light guide for each color. These light guides may take the form of three overlying plastic or glass sheets. In another embodiment, thin fiber optic cables arranged parallel to each other on a supporting surface are used as the light guides, and each fiber optic cable is optically coupled to only one R, G, or B LED in a repeating RGB pattern. The light guides contain deformities coinciding with the positions of the red, green, and blue pixels. These deformities may be any of those used in the prior art for xe2x80x9cleakingxe2x80x9d light out of the light guide. The R, G, and B LEDs are constantly on, and there is no color filtering.
In one embodiment, the LCD has red pixels arranged in a column, green pixels arranged in an adjacent parallel column, and blue pixels arranged in a column adjacent to the green pixels. The pattern repeats. For this type of display, the deformities on each of the light guides are arranged in strips coinciding with the columns of pixels for the particular color transmitted by the light guide. For this type of embodiment, fiber optic cables as light guides are particularly suitable since the light of a particular color is inherently restricted to a column by the fiber optic cable. The thickness of each cable is approximately equal to the width of a pixel.
Since the inventive backlighting technique allows the RGB LEDs to be on 100% of the time, unlike the technique shown in FIG. 2, and no RGB pixel filter is required, unlike the technique shown in FIG. 1, the LCD uses much less energy to provide the same brightness as the prior art displays.